Duration.js

(function(){

var re, units = {ms: 1, s: 1e3};

Duration = function(value){
	return Duration.parse(value);
};

Duration.parse = function(value){
	var match = value.toString().match(re);
	return match ? parseFloat(match[1]) * (units[match[2]] || 1) : parseFloat(value)
};

Duration.addUnit = function(keys, multiplier){
	Array.from(keys).forEach(function(key){
		units[key] = multiplier;
	});
	re = RegExp('([\d.]+)(' + Object.keys(units).join('|') + ')');
};

// init
Duration.addUnit([]);

})();

console.log(Duration(200));
console.log(Duration('200'));
console.log(Duration('200ms'));
console.log(Duration('.2s'));